Parker Pressure Compensation Directional Control Valve - VPL

Parker Pressure Compensation Directional Control Valve - VPL

The importance of selecting control valves
Control valves are actuators in self-control systems, and their application quality is directly reflected in the system's regulation quality. As a terminal component in process control, people have a renewed understanding of its importance compared to the past. The quality of the application of regulating valves depends not only on the quality of the product itself and whether the user has installed, used, and maintained it correctly, but also on the correct calculation and selection. Due to errors in calculation and selection, the system may start and stop, and some may even be unable to be put into use. Therefore, users and system designers should recognize the importance of valves on site and pay sufficient attention to the selection of regulating valves.
Principles for selecting control valves
1. Choose the appropriate structural form and materials according to the process conditions.
2. Select the flow characteristics of the control valve based on the characteristics of the process object.
3. Select the appropriate control valve diameter size based on the process operating parameters.
4. Select the required auxiliary devices according to the requirements of the process.
5. Reasonably choose the executing agency. The response speed of the actuator should be able to meet the process requirements.
Types and selection of control valves in the power industry
1. Main and auxiliary boiler feedwater valves, composite feedwater regulating valves:
Control requirements: Control the boiler water inlet, maintain the boiler liquid level and steam flow rate.
Working condition requirements: When the boiler starts, the valve pressure difference can reach 100-300 kilograms, with a small flow rate. During normal operation, the pressure difference is small, with a large flow rate, and the flow rate can be adjusted to a ratio of 75:1-100:1.
Valve requirements: The valve body material is WCB/WC9. For the starting valve in the dual valve system, it is required to be anti cavitation and have a V-level closure level. For the composite feedwater valve, characteristic valve internals should be used to meet the needs of both boiler start-up and normal operation. Generally, valves are selected for fault closure. (It should be noted that hydrazine is added to the water supply to eliminate dissolved oxygen, which can cause adverse reactions to SATELLITE 6 alloy or other similar alloys. Therefore, instead of using 316 STELLITE welding to treat valve internals, 416 stainless steel material can be directly used.).
2. Boiler feedwater pump small flow circulation valve:
Control requirement: Ensure the safe start-up and operation of the boiler feedwater pump, and eliminate pump damage caused by low flow rate, increased temperature rise, and strong water cavitation. (Old style power stations use switch type small flow control valves, as well as mechanical pump automatic circulation valves).
Operating conditions: The pressure drop reaches 160-350 kilograms, making it the valve in the power station system that can withstand large pressure differentials. The flow rate is generally 30% of the normal flow rate.
Valve requirements: anti cavitation, anti blocking structure, V-grade sealing level. The valve should work continuously when the pump starts and stops running. After the pump starts normally, the valve should be closed. The valve is usually opened due to a fault.
3. High/low heater water supply heater drain, condensate water heater drain:
Control requirement: Control the position of condensed water in the heater, promptly discharge the condensed water, i.e. discharge at high water level and drain at normal water level.
Operating conditions: The medium is saturated water, and the pressure difference between the control valves of the heaters is generally 6-30 kilograms. The downstream drain valve is directly connected to the condenser or deaerator.
Valve requirements: anti flash erosion, V or VI sealing level, valves are usually opened in case of malfunction.
4. Deaerator water level control valve (DALC):
Control requirement: Provide flow to maintain the normal water level of the deaerator, and eliminate dissolved gases in the water by adding steam to increase the water temperature.
Operating conditions: During the start-up phase, the flow rate is small and the pressure difference is large. Under normal circumstances, the flow rate is large and the pressure difference is small. The operating conditions are similar to those of a water supply valve.
Valve requirement: Effective resistance to cavitation at lower flow rates.
5. Temperature reducing valve (thermostat):
Control requirement: Spray high-pressure water into the steam to achieve the required temperature control.
Operating conditions: The medium is water with a large pressure difference. Valve requirements are diverse: nozzle type, auxiliary atomization type, self containment type, and clamp type.
6. Temperature and pressure reducing valve (high and low pressure bypass valve):
Control requirement: Adjust high-temperature and high-pressure steam to a certain temperature and pressure of steam; Provide a bypass channel for the steam turbine, allowing the boiler to operate independently of the turbine.
Working condition: High temperature and high pressure steam temperature and pressure reduction.
Valve requirements: resistance to thermal shock, noise reduction, and rapid response.
Requirements for Accessories in Selection
In the production process, the control system puts forward various special requirements for valves, therefore, control valves must be equipped with various accessory devices (referred to as accessories) to meet the needs of the production process. The accessories of the control valve include:
1. Valve positioner is used to improve the working characteristics of control valve regulation performance and achieve correct positioning;
2. The valve position switch displays the working position of the upper and lower limit stroke of the valve;
3. When the air source of the control valve fails, the pneumatic holding valve maintains the valve in the open position before the air source failure occurs;
4. Electromagnetic valves achieve electromagnetic switching of the gas path, ensuring that the valve is in the desired safe opening position in case of power failure;
5. When the controller of the control system fails, the handwheel mechanism can switch to manual mode to operate the valve;
6. Pneumatic relay accelerates the action of the actuator and reduces transmission time;
7. Air filter pressure reducing valve is used to purify the air source and regulate the air pressure;
8. The air storage tank ensures that in the event of a gas source failure, the springless cylinder piston actuator can move the control valve to the fault safe position. Its size depends on the size of the cylinder, the requirements for valve action time, and the working conditions of the valve;
In short, the function of attachments is to make the control valve more efficient, reasonable, and complete.

Parker Pressure Compensation Directional Control Valve - VPL